Radio receiving system



Feb. 23, 1931,

R. M. SMITH RADIO RECEIVING SYSTEM I Filed- Dec. 29, 41934 INVENTOH M.ih

Rogers Patented Feb. 23, 1937 STATES RADIO RECEIVING SYSTEM Rogers M.

to Radio Corporation of America,

of Delaware Smith, Merchantville, N. J., assignor a corporationApplication December 29, 1934, Serial No. 759,629

19 Claims.

My invention relates to radio receivers and more particularly toreceivers used for broadcast reception in motor driven vehicles, such asautomobiles and airplanes.

Heretofore it has been common practice in radio receivers to provide anantenna input circuit that resonates at a frequency outside of thebroadcast range to which the receiving circuit is tunable. Where theantenna circuit has been provided with a large transformer primary coil,and resonant at a frequency below the range in order to boost the gainat the lower end of the range to compensate for non-uniform sensitivityof the amplifying stages, interference has occurred in some casesbecause the antenna circuit responded to waves outside the range. Incertain receivers of simplified design the interference was particularlydisturbing where the intermediate frequency amplifier in asuperheterodyne receiver tuned somewhere near the frequency ofinterfering long waves below the range.

A serious source of interference in automobile and airplaneinstallations has been the high frequency transient currents set up bythe engine 25 ignition system. In conjunction with the above mentionedlarge primary circuits it has been customary to provide some capacitycoupling between antenna and receiver input circuits to increase gain atthe higher end of the range. I have found that such capacity couplingprovides a ready path for these objectionable ignition disturbances. Forthe purpose of reducing this interference it has been customary in thecase of automobiles to use suppressor resistors in series with eachignition cable. In airplane installations, where there are seriousobjectionsto the use of such suppressors, it has been necessary toresort to very elaborate and expensive shielding of ignition andreceiver apparatus.

Accordingly, it is an object of my invention to provide an antennasystem for a radio receiver for use on motor driven vehicles, whichminimizes the above mentioned interference and makes unnecessary theextensive use of suppressor resistors and shielding.

A further object of my invention is to provide an antenna. circuit,having fixed band-pass filter characteristics, fora radio receiverwherein the plurality of succeeding tunable circuits may be readilyadjusted by means of a common control for tuning to a desired signalfrequency without substantial deleterious eilect by the antenna circuitupon the tuning characteristics of the tunable circuits.

It is a further object of my invention. to provide an antenna-inputsystem for a radio receiver which shall discriminate against allinterfering waves outside the range of frequencies utilized by the radioreceiver, and which shall have a desirable uniform voltage gain for theentire frequency range utilized by the receiver.

A still further object of my invention is to provide an improved antennaand lead-in structure for reducing ignition disturbance.

Other objects of my invention will become evident from the followingdetailed description taken in conjunction with the accompanying drawingin which In Fig. 1 there is illustrated diagrammatically a preferredmethod of carrying the invention into effect,

Fig. 2 is a schematic diagram of a specific form of my invention asadapted for installation in an automobile,

Fig. 3 is a simplified circuit diagram shown for the purpose ofillustrating the underlying principles of my invention,

Fig. 4 illustrates characteristic curves showing the voltage responsecharacteristic of the antenna-input system over the broadcast frequencyrange, the curves being in terms of voltage gain versus frequency, and

Fig. 5 is a side elevation view, partly in section, of a modified formof my invention.

In accordance with my invention I have provided a filter, as of thelow-pass type, connected between the circuit of a collector of signalenergy, such as a capacity type antenna and ground or counterpoise, andthe input circuit of a radio receiver, such as a superheterodyne. Theelectrical relations between the filter and associated circuits are suchthat a substantially fixed bandpass filter arrangement is set up aheadof the tunable selective input of the receiver, whereby objectionableinterference is substantially reduced, while preserving the usualdesirable antenna-input circuit signal voltage gain over the tuningrange.

Further in accordance with my invention I have provided for use on amotor vehicle, animproved signal collector arrangement and shieldedlead-in structure which, together with the above mentioned filter,substantially eliminates ignition interference set up by transients,making unnecessary the customary extensive use of suppressor resistorsand elaborate shielding.

Referring to Figs. 1 and 3, G refers to a source of signal energy, 01 toan effective series capacity, known as the antenna to ground capacity;C2 to a shunt capacitor that functions as a by-pass capacity forinterference as well as for another purpose that will be hereinafterdescribed; L11, a series inductance coil, or reactor, that serves aspart of a filter; and shunt capacitor C3 which forms, with C2 and L11, alow-pass type filter. In an actual circuit that I have designed andbuilt in accordance with my invention, for use on automobiles, C1, byway of example, was around 100 mmf. (micro-microfarads), C2 of the orderof 100 to 200 mmf., C3 around 200 mmf. and L11 around 290 microhenries(about 105 turns, lumped, on one-half inch form). The elementscomprising the filter are shown enclosed in dash lines S15 and markedFilter, the lines representing a grounded shield enclosing the filterelements.

The output of the filter is connected to an antenna transformer primaryL12, the secondary L13 of which is loosely coupled thereto and is tunedby a variable condenser C14 throughout a range of frequencies. Thesignal input transformer L12-L13 is preferably enclosed within agrounded shield S32, as indicated in dash lines. In an actual circuitthat I have designed inaccordance with my invention, primary L12'has avalue of approximately 50 microhenries (52 turns on a half inch diameterform), and secondary L13 has approximately 2'75 microhenries inductance(about 188 turns, sectionalized, on same form as primary). It is notedthat secondary L13 in the input of the first stage of the set, has lessactual inductance than that of the corresponding secondary coil (Ln-Fig.2) in the following stage, measured in a manner to eliminate the eifectsof associated circuits. The coupling between coils L12 and L13 is aroundin the above example.

The filter circuit serves two purposes, one for attenuating allfrequencies other than the band of frequencies over which the receivingsystem tunes, and the other for effecting a high uniform voltage gain inthe antenna-input system. The proper selection of values for condenserC2, the antenna capacity 01 being considered, and of inductance L11,governs more directly the gain at the low end of the band, while thevalues of capacitor C3 and inductance L12 control more directly the gainat the higher end of the band. See curve A in Fig. 4, representingvoltage measured across the secondary of the signal input transformerover the frequency range.

In Fig. 1, I have illustrated more completely, in a schematic circuitdiagram, the details of an embodiment of my invention. The antenna andground, or collector circuit, designated as such on the diagram, areconnected through a shielded cable 2! to a radio receiver. The antennato ground capacity is indicated in dotted lines at C1, corresponding toC1 in Fig. 3. The leads 23 and 25 from the antenna and ground terminals21 and 29, respectively, constitute a balanced transmission line and arepreferably twisted to effect a transposition against pickup, asindicated diagrammatically, and are further protected from disturbinginterferences by means of a flexible sheath or shield 28 suitablyconnected to ground at terminal 29.

For the purpose of materially reducing interference, such as ignitionnoises in the case of installation in an automobile, the ground terminalis made at a point 29 that is close to the antenna terminal 21 for thereasons set forth in Perkins Patent No. 1,943,394, January 16, 1934,disclosing and claiming this arrangement. Whereas Perkins uses theground lead as a shield for the antenna 1ead-in, I have found thatbettershielding shield against disturbances.

results are obtained by using a shield that is separate from the antennaand ground conductors or lead-ins. The receiver end of the conductorshield 28 is electrically connected to the metal case S24 enclosing theradio receiver chassis, as indicated in Fig. 1 and in more detail at 4|in Fig. 2. For best results, the conductors should not be exposed atthis point. The circuit shown in Fig. 1 has beneficial results when usedin home receivers, particularly in the case of bad local interference,e. g. electrical appliance disturbances. For the automobile installationthis subject matter is taken up more fully in connection with Fig. 2.

In automobile receivers it is customary to enclose the receiver chassisin a metal case that serves both as a housing and as an electrical Theterm chassis base has been commonly employed to designate the customarysheet material base or frame work on which the various circuit elementsare assembled. However, the term chassis is commonly used to include thevarious circuit elements in combination with the chassis base as acomplete unit assembly. The case, the chassis and the lead-in cableshield are all electrically connected at the same RF potential andconstitute what may be broadly viewed as a receiver structure ofelectrically conductive material.

Condenser C2 in Fig. 1 should be of the trimmer type for adjustment atthe time of installation in home receivers where the antennas varywidely in their characteristics with different cases. This is for thepurpose of matching up the antenna with the system in order to obtainthe proper response characteristic, as illustrated in Fig. 4, and toinsure that the ganged tuning condensers shall be substantially inalignment for tracking throughout the range. Adjustment of condenser C2substantially affects the alignment of the secondary circuit L13C14 atthe low frequency end of the range. Electrically, the distributedcapacity C2, in dotted lines, between the conductor lead-ins 23, 25 andbetween the lead-ins and the sheath shield 28, indicated in dotted linesat C4 in Fig. l, effectively in series across the line, are consideredto be a part of the adjustable trimmer condenser 02, and together thesecapacities correspond to C2 in Fig. 3. Where the circuit is desired foruse in home receivers, where the capacity of different antennas variesover a wide range, for example, more than 50 mmf., it is desirable tomake C2 variable, as above outlined, in order to adapt it to aparticular installation.

For automobile installations where the antenna capacity is relativelyquite large, and of a substantially known uniform value for differentinstallations, it is not necessary to use an actual condenser 02, infixed or variable form, in addition to thedistributed capacities C2 andC4. Inherently, these distributed capacities provide a substantialdegree of by-pass action for high frequency interference or parasitics.For further reduction of ultra high frequency interference it isdesirable in some cases to employ a lumped by-pass means, such as splitcapacitors C5, connected between the conductors 23, 25 and the metalcase of the receiver, in such manner as to not disturb existing balancedrelations.

For the purpose of improving the band-pass characteristic of the curve,to give a flatter top and to reduce the peak at the low frequency end ofthe range, I have inserted a resistor R in series with the inductancecoil in the filter circuit. I

have shown the resultant curve in dash lines in Fig. 4, as curve B.Resistor R has the effect of reducing the transfer of signal energy fromthe primary to the secondary circuit for frequencies near the lower endof the range. The insertion of resistor R decreases the effect ofcapacity C2 on the secondary circuit; increasing the resistance of Rsubstantially decreases any detuning effect caused by variations thatmay occur in capacity of C2. By way of example, in a system constructedin accordance with my invention, the resistance of R is 100 ohms. Thisresistance may be introduced by winding L11 of resistance wire of thedesired kind.

It will be noted that the shield S15 around the filter is electricallyconnected to the metal case of the radio receiver at 39. The inputtransformer L12L13 is likewise enclosed in a shield S32 that is groundedto the metal case. An electrostatic shield S33, of the type shown inShapiro Patent No. 1,942,575, may to advantage be interposed between thecoils of the transformer L12- L13 for substantially eliminating allcapacity coupling between the antenna-system and the receiver. Inpractice, however, I have found that sufficiently satisfactory resultsmay be obtained by taking precautions to reduce capacity couplingbetween primary and secondary, as by placing the primary at the low endof the secondary and running the leads so as to reduce capacitycoupling. See Fig. 2. In certain installations where ignitioninterference is severe, however, it is sometimes desirable to employ thecapacity shield, along with split condensers C and C5, above mentioned.

Referring more particularly to Fig. 2, I have shown my novel system inits application to an automobile set installation. In accordance with myinvention, I have found that excellent results may be obtained with anantenna 35 made from bending a metal tube or pipe, into the shape of a Uor hairpin and mounting this on suitable insulators 3i underneath therunning-board 99, or underneath the chassis. By way of example I used athree-eighths inch 0. D. metal tubing, bent into a U form about fourfeet long with a separation of five inches between the legs of the U. Iam quite aware of the fact that a metal plate has been used under anautomobile for an antenna (RCA-Victor model M-30 of 1931). The advantageof my U-shaped, tubular construction is that it materially reduces thecapacity without a corresponding sacrifice in pickup ability. This formof antenna, mounted underneath a car, is particularly advantageous inthe case of all-- metal tops used on some cars. The antenna is mountedby means of metal straps 39 bolted to metal supports on the underneathside of the running-board, about three to six inches below, by way ofexample. Rubber bushings or grommets 37 are carried by the lower ends ofthe straps 39 for directly supporting the tubular antenna structure ininsulated relation with respect to the car frame or chassis. The antennalead-in is attached at or near the rounded end of the tubular antenna at27 and the ground connecgrounded at or near the same point as the groundconnection 29. For the purpose of completing the A battery circuit tothe heaters of the tubes, shown in the diagram, the metal case S24 ofthe radio set is preferably also connected electrically to the metalframe of the car at some point 29a adjacent its location on theautomobile. Capacity C2, shown in dotted lines, representing deflnitecapacity inherent in the cable design, corresponds to C2 in Figs. 1 and3. As stated above, itis not usually necessary to use an actualcondenser in an automobile installation.

Only the first and second tuned circuits of a radio receiver have beenshown for purposes of illustration, Fig. 2. L13, C14 constitute thetuned input of a radio frequency amplifier stage, while L17, C18constitute the tunable input to the first detector of a superheterodynereceiver, the condensers C14 and C18 being ganged for single controloperation. The output of the thermionic amplifier I9 is coupled to coilL17 by means of primary coils L16.

By way of example, primary L16 is of about 650 turns, the largeinductance type of primary, made in accordance with the teachings ofpatents to Beers, 1,907,478 and 1,973,037. The primary circuit resonatesbelow the range of frequencies to which the secondary circuit tunes.

Any satisfactory means may be used for attaching the shielded cable tothe radio set housing. I have employed a flange 4| on the end of thecable, with a screw 43 for fastening the cable shield to the set casingto insure a good shielding connection. If desirable, a plug connectionmay be employed so that the cable is readily detachable for servicepurposes. The elements of the filter and the primary L12 of the first RFtransformer may be included within a shielded plug 45, as shown in Fig.5, provided with any convenient means of attachment such as a bayonettype lock means 48 and 49, for insertion into an opening in-the receiverhousing. The primary L12 is adapted to be inductively coupled to thesecondary L13 when the plug is in position. When in position therelations of elements are substantially the same as above. Electrostaticshield S33 is preferably used with this arrangement for the purposes ofminimizing capacity coupling.

By application of my invention, I have found, as a result ofinstallations in several different makes of automobiles, that ignitiondisturbances have been substantially eliminated. The reduction iniginition noise was so great that suppressor resistors in the ignitionsystem were not required. In certain bad cases of ignition interferencecom pletely satisfactory results were obtained with the use of but onesuppressor resistor in the main ignition cable, between the transformerand the distributor. In no case was it necessary to employ amultiplicity of suppressors attached tothe spark plugs. This is of amaterial advantage in that suppressor resistors have added to ignitiontroubles and expense of installation, and are seriously objected to bymany, particularly in the case of aviation ignition. In the case ofradio receiver installation in airplanes the necessity for elaborateshielding has been greatly reduced by the employment of my invention.

An additional desirable advantage of my invention is that imagefrequency interference, encountered in some superheterodyne receivers,is substantially reduced.

While I have described certain forms that my invention may take, itshould be obvious to anyone "skilled in the art that my invention is notlimited to these embodiments.

I claim as my invention:

1. The combination with a radio receiving system on a motor drivenvehicle of an input circuit tunable to select desired frequenciesthroughout a broad tuning range and including a transformer having aprimary and a tunable secondary; a. collector of signal energy; a filterof substantially fixed nature for suppressing ignition interferenceconnected between said collector and said input circuit and comprisingsaid primary, a condenser in shunt therewith, and an inductance coil inseries between the primary-condenser combination and said collector;said filter in combination with said collector having a substantialresponse to signal energy within said range.

2. The combination with a radio receiving system of an input circuittunable to select desired frequencies throughout a broad tuning rangeand including a transformer having a primary and a tunable secondary; acollector of signal energy; a filter of substantially fixed natureconnected between said collector and said input circuit and comprisingsaid primary, a condenser in shunt therewith and tuning with the primaryto a frequency of the order of the upper end of said range, and aninductance coil in series with said primary-condenser combination andsaid collector; said filter in combination with said collector and saidinput circuit having a bandpass characteristic with an upper cut-offadjacent the higher end of said range whereby interference atfrequencies above said range is substantially eliminated.

3. The combination with a radio receiver on a motordriven vehicle of aninput circuit tunable to select desired frequencies throughout a broadtuning range and including a transformer having a primary and a tunablesecondary; a collector of signal energy; a shielded lead-in connectedbetween said collector and said receiver; a filter of substantiallyfixed nature for suppressing ignition interference connected betweensaid collector and said input circuit comprising said primary, acondenser in shunt therewith, and an inductance coil in series with saidprimarycondenser combination and said collector; said filter incombination with said collector and lead-in capacity and said primaryhaving a band-pass characteristic with a resonance point adjacent thelower end of said tuning range,

4. In a radio receiving system, a collector of signal waves, an inputcircuit including a transformer primary coil and a secondary coiltunable throughout a frequency range, said primary coil having a naturalresonant frequency above said frequency range, a filter connectedbetween said collector and primary coil for eliminating disturbininterference, said primary coil tuning with a portion of said filterwithin said range adjacent the higher end thereof.

5. The invention as set forth in claim 4 characterized in that anotherportion of said filter in combination with said collector tunes withinsaid range near the lower end thereof.

6. The invention as set forth in claim 4 characterized in that anotherportion of said filter in combination with said collector resonates nearthe lower end of said frequency range, and resistance means in saidlast-named portion for reducing and broadening the resonant response atsaid lower end.

7. The combination with a radio receiving system of an input circuitincluding a transformer tunable to select desired frequencies throughouta broad tuning range, an antenna circuit including a filter ofsubstantially fixed nature connected between antenna and said inputcircuit, said filter comprising a resistor and inductance coil connectedin series between the antenna and input circuit and a capacitorconnected in shunt with the primary of said transformer, said primaryand said condenser together tuning to a band of frequencies near thehigher end of said tuning range, said inductance coil and antennacapacity having a natural period within said tuning range adjacent thelower end thereof, and said resistor serving to modify the responsecharacteristic of said last named natural period.

8. The invention as set forth in claim '7 characterized in that anadjustable condenser is provided in shunt with said antenna capacity foradjustments of semi-fixed nature in installations where the antennacapacity may be of some value within wide limits.

9. An antenna-input system for a radio receiver including a structure ofelectrically conductive material and mounted on a motor driven vehiclehaving a frame or counterpoise of electrically conducting material,comprising an antenna supported adjacent said frame, a ground orcounterpoise terminal on said frame adjacent said antenna, asubstantially balanced transmission line including a pair of lead-insconnected from said antenna and said ground terminal, respectively, tosaid receiver and characterized by freedom from unbalancing groundconnections at the receiver end of said line, and a filter connected incircuit between said line and said receiver for suppressing interferencepicked up by said antenna.

10. The invention as set forth in claim 9 characterized in that meansare provided for bypassing parasitic frequencies from said lead-inswithout disturbing the electrically balanced relation to saidelectrically conductive structure of said receiver.

11. The invention as set forth in claim 9 characterized in that saidlead-ins at said receiver are by-passed to the electrically conductivestructure of said receiver through a split condenser arrangement forreducing ultra high frequency interference.

12. The invention as set forth in claim 9 characterized in that meansare provided for minimizing capacity coupling between said antennatransmission line and said receiver.

13. In an antenna input circuit for a radio receiver on an automobile, aU-shaped antenna structure supported beneath the running-board of saidautomobile, an input circuit comprising a transformer tunable throughouta broad range of frequencies, a shielded transposed transmission lineconnected from said antenna structure to said input circuit, means forminimizing capacity coupling between said line and said input circuit,and a filter connected in circuit between said plug, plug with saidradio receiver, the secondary of said transformer being mounted in saidcase adjacent said socket whereby said coils are coupled in operativerelation when said plug is inserted in said socket.

15. The invention as set forth in claim 14 characterized in that afilter is connected in circuit with said antenna and is mounted withinsaid plug.

16. The invention as set forth in claim 9 characterized in that saidantenna structure comprises a U-shaped metallic tube.

1'7. In a tuned radio receiving system a plurality of circuits tunablethroughout a frequency range by a common control, an antenna circuitincluding a filter of substantially fixed nature and having, with saidfilter, a band-pass response over the tuning range, a transformer forcoupling said filter and antenna circuit to said receiver, a thermionicstage connected to the secondary of said transformer, a secondthermionic stage connected in cascade to said first named stage througha transformer, the actual inductance of said first named secondary beingless than the actual inductance of the secondary of said secondtransformer whereby said circuits tune in like. manner over thefrequency range.

18. "In a tuned radio frequency receiving sysmeans for detachablyconnecting said v tem a plurality of thermionic stages in cascade, saidstages being tunable throughout a range of frequencies by means of acommon control, an antenna circuit having means for causing said circuitto have a substantially uniform gain with a band-pass characteristicover the tuning range, a transformer for coupling said antenna to thefirst stage, a second transformer for coupling the output of the firststage to the input of the succeeding stage, the primary circuit of saidtransformer being resonant below the tunable range, the secondary actualinductance of said first transformer being less than the actualinductance of the secondary of said second transformer.

19. In a radio receiving system, a collector of signal waves, an inputcircuit including a transformer primary coil and a secondary coiltunable throughout a frequency range, said primary coil having a'naturalresonant frequency above said range, a low pass filter connected betweensaid collector and primary coil for eliminating disturbing interference,said filter including a series connected inductance coil having anatural period above said frequency range, said filter together withsaid collector and primary coil having a band-pass characteristic withsubstantially uniform gain over said frequency range.

ROGERS M. SMITH.

